We use Abstract class and interface to enforce some rules to the classes which extends/implements. For example we can define a class say "Bird" and we can declare methods say "Fly()", "Walk()". This means whatever the class that is derived from Bird has to override or give definition for Fly() and Walk() and therefore we are making sure that all the derived classes follows or has the common functionalities. In other way the classes derived from superclass should have common properties. In addition to this the derive class can have its own methods like "Swim()"...

In case of Abstract class we can define COMMON functionalities in super class and those can be used in the derived class where as in Interface we cant do that. ( this i would say as advantage of abstract class)

In case of Interface the derived class can implement any number of interface but restricted to extend only one abstract class (this i would say as advantage of Interface)

Thanks

Prasanna